Field of the Disclosed Subject Matter
The presently disclosed subject matter relates to methods of preparing oligomers of an olefin. For example, the presently disclosed subject matter provides methods of preparing 1-hexene from ethylene. The methods can include the use of particular hydrocarbon solvents, e.g., n-undecane and/or a C8-C11 alkane compound having one branch.
Description of Related Art
Oligomerization of olefins can produce many valuable chemical products. For example, simple feedstock olefins can be oligomerized to provide useful higher olefins. Industrially important processes include the preparation of alpha olefins (α olefins) from ethylene, e.g., the preparation of 1-hexene via trimerization of ethylene.
A problem encountered in some oligomerizations of olefins can be incomplete selectivity for the desired oligomer. That is, in addition to the desired oligomer, some side products and byproducts can be formed. The side products and byproducts can include other oligomers as well as polymers. As an example, in oligomerization of ethylene to 1-hexene, other isomers of hexene (e.g., 2-hexene and 3-hexene) can be formed, as well as higher oligomers of ethylene (e.g., octene, decene, and dodecene) and polyethylene.
Polymers formed as side products and byproducts during oligomerization of olefins can be problematic. Polymer that remains dissolved in a product mixture must be separated from the desired oligomer product. Moreover, polymers can impair process performance. For example, on industrial scale, polymer can deposit on internal parts of oligomerization reactors and/or product pipelines as well as other equipment that comes into contact with reaction and product mixtures. Removal of polymer deposits can require shutdown of process equipment for days or weeks and can require substantial effort, which can include steaming of the polymer, treatment with water to peel the polymer from equipment surfaces, and physical removal of the polymer.
As an example, in oligomerization of ethylene to 1-hexene, polyethylene can deposit on reactors and downstream equipment and can complicate purification of 1-hexene. It can therefore be beneficial to discourage deposition of polyethylene and to facilitate removal of polyethylene from a product stream from an ethylene trimerization reaction.
Olefin oligomerization reactions can be conducted in hydrocarbon solvents. For example, in some existing processes for preparation of 1-hexene from ethylene, C6-C7 alkanes (including cycloalkanes) are used as solvents. Known solvents include cyclohexane, methylcyclohexane, and heptane. Such solvents can have advantageous properties (e.g., good solubility of organometallic catalysts) but also tend to have boiling points similar to that of 1-hexene, which can make separation of 1-hexene from the solvent difficult.
Various attempts have been made to use other hydrocarbon solvents in olefin oligomerization reactions. For example, U.S. Patent Publication No. 2013/0144024, European Patent No. EP0668105, Chinese Patent Application Publication No. CN102558107, Chinese Patent Application Publication No. CN103102237, European Patent Application Publication No. EP2098543, and U.S. Patent Publication No. 2013/0102826, each of which is hereby incorporated by reference in its entirety, variously describe use of linear and branched C5-C12 alkane solvents for preparation of 1-hexene from ethylene. However, the C8-C11 branched alkanes described in the prior art as solvents for olefin oligomerization reactions are alkanes with multiple branches, e.g., isooctane (2,2,4-trimethylpentane). The prior art generally suggests that higher (C8 and above) alkane solvents have inferior solubility properties as compared to C6 and C7 solvents. Furthermore, the prior art generally suggests that cyclic alkane solvents are better able to dissolve organometallic catalysts and side product polymers than linear hydrocarbons, and that linear hydrocarbons are better able to dissolve organometallic catalysts and side product polymers than branched hydrocarbons. See also M. P. McDaniel, Advances in Catalysis, 2010, Vol. 53. Changes in solvent composition are known to have unpredictable effects on catalyst activity, as described in U.S. Pat. No. 7,718,838, which is hereby incorporated by reference in its entirety.
There remains a need for methods of preparing oligomers of an olefin with reduced deposition of polymer side products and easier separation of polymer from oligomer products. It is therefore desirable to provide solvents for olefin oligomerization that achieve improved catalyst solubility, improved catalyst activity, improved polymer solubility, improved separation of polymer, and improved separation of oligomer product.